The present invention relates generally to alloy steels, and, more specifically, to martensitic steel alloy compositions intended for use as high speed tool steels, hot work and cold work die steels, armor plate, and other applications requiring good response to hardening, resistance to softening at high work temperatures, and high yield strength, the alloy composition being characterized by the presence of intermetallic compounds and precipitates that are substituted for cobalt.
A conventional approach to obtaining the hot hardness properties required of alloy steels useful for applications as described above has been to use cobalt as an alloying element. Cobalt is an expensive, strategic material that must be imported. In some applications, such as high speed tools, alloying amounts of cobalt are thought by some to have detrimental health effects. Cobalt additions in heat resisting steel compositions also have certain negative effects, such as reduction of material toughness. Because of these considerations, various attempts have been made to eliminate cobalt by substituting alloy systems based on carbide forming elements, such as niobium, titanium, chromium, tungsten, molybdenum and the like. A study was also made of the possibility of using silicon and aluminum additions totaling about 2.5% in order to replace or substantially reduce cobalt in high speed tool steels. One reason that none of these expedients have been commercially accepted is believed to be the inability to consistently achieve the desired hardness and strength properties and/or irregular results.,
The purpose of the present invention is to eliminate or significantly reduce cobalt in heat resisting steels, such,as high speed tool steels (HSS), die work steels, armor plate and the like, without sacrificing, and in many instances improving the properties expected of cobalt systems, particularly response to hardening and resistance to softening at elevated temperatures.
In general, the purpose of the invention is accomplished by substituting for cobalt sub-micron and nano-structural precipitates and intermetallic compounds including M3Si. The formation of these precipitates and compounds is promoted by the addition of each of nickel, copper, aluminum, manganese and silicon in a total amount of at least 4.0% by weight with an optimum minimum amount being 4.5%. The silicon content exceeds 1.0%. The addition of these elements improves material response to hardening, resistance to softening at elevated temperatures and yield strength in all martensitic grade steels at very low cost in comparison to cobalt alloyed steels. The resistance to softening at elevated work temperatures is so significant that it may exceed that of super cobalt alloyed HSS materials.
xe2x80x9cAs used in the following disclosure and in the appended claims, all indicated percentages are to be understood to mean percentages by weight.xe2x80x9d
One embodiment of the invention is a martensitic alloy steel consisting essentially of about 0.15-3.5% C, 0.5-13.0% Cr, 0.05-15.0% V, 0.75-12.0% Mo, 0-15.0% W, a residual amount of Co less than 1.25%, more preferably, 0.5% or less, each of Ni, Cu, Al, Mn and Si in a total amount of at least 4.0% with the silicon content exceeding 1.0%, and the balance essentially iron.
A more specific embodiment of an HSS material within the scope of the invention having the desired properties of good hardening response, resistance to softening at elevated work temperatures and good yield strength is a martensitic alloy consisting essentially of about 0.15-1.15% C, 3.5-4.5% Cr, 1.0-1.6% V, 8.5-10.0% Mo, 1.4-2.10% W, less than 1.5% Co, each of Ni, Cu, Al, Mn and Si in a total amount of at least 4.0% with the silicon content exceeding 1.0%, and the balance essentially iron.
Still another example of the invention is an alloy system useful for armor plate including 0.15-0.35% C, 0.60-1.00% Mn, 0.40-0.90% Cu, 7.0-10.0% Ni, 0.50-1.00% Al, 1.0-1.50% Si, 0.05-0.25% V, 0.50-1.25% Cr, and 0.75-1.25% Mo.
Examples of hot work and cold work die steels contain 0.30-3.5C, 3.50-13.0% Cr, 2.75-15.0% V, 0.75-2.00% Mo, 5.75-6.75% W, less than 1.25%, and more preferably less than 1.0% Co, 0.70-1.20% Ni, 1.0-1.75% Si, 0.50-1.50% Al, 0.40-0.90% Cu, 0.60-1.00% Mn, and the balance essentially iron.
In carrying out the invention, a steel according to any of the previous paragraphs is austenitized in the range of from 1750-2250xc2x0 F., rapid quenched to room temperature, and multiple tempered to a range of from 900-1050xc2x0 F.